Datafield Works in Sim but not on 920xt

Created a very simple datafield using Eclipse/MacOSX

using Toybox.WatchUi as Ui;
using Toybox.Application as App;

class HalfMarathonView extends Ui.SimpleDataField {

var DIST = 21.1;

//! Set the label of the data field here.
function initialize() {
label = "1/2 Mar To Go";
}

//! The given info object contains all the current workout
//! information. Calculate a value and return it in this method.
function compute(info) {
// See Activity.Info in the documentation for available information.
if(info.elapsedDistance == null) {
return DIST;
}
else {return DIST - (info.elapsedDistance/1000);
}
}

}

Works fine in the sim but when I copy the prg file to the Garmin/Apps folder I can see it under the watch settings on my 920xt but selecting it does nothing ie the field remains as the previous data field.

I assume there is some kind of validation checking being done on the 920xt itself ?